Method and apparatus for locating non-visible objects

ABSTRACT

Non-visible objects which differ in their physical properties from their surroundings by association with a variable strength magnetic field may be detected by a suitable array of Hall effect sensors which can be moved relative to the object in question. By analysing the signals from the plurality of the sensors in the array, the position of the object can be deduced relative to the array and the array moved to enable a machining guide thereon to be aligned with the non-visible object. The system is of particular value in locating apertures ( 2 ) in wing spars ( 1 ) when attempting to fix the skin ( 3 ) of the wing on to them where it is important to be able to locate the correct point at which to drill a hole through the skin ( 3 ) to coincide with the hole ( 2 ) in the spar. By locating a magnet ( 4 ) relative to the hole to identify the hole magnetically and using an array of Hall sensors ( 12 ) in a base with an aperture ( 14 ), it is possible to shift the array ( 10 ) so that the aperture ( 14 ) is precisely aligned with the non-visible hole ( 2 ).

This invention relates to locating non-visible objects, particularlythough not exclusively for the purpose of identifying the position of anon-visible object prior to carrying out a mechanical processing step inthe vicinity of the object so located.

There are many situations where it is desired to locate somethingaccurately, although the item in question is not visible. A simpleexample is to locate the position of a load-bearing member in apartition wall made of a wooden frame to either side of which sheets ofplasterboard are attached. If it is desired to fix something to thewall, e.g. using a hook, it is necessary to ensure that the hook, e.g.screwed into the wall, goes into part of the timber support rather thaninto the plasterboard, from which it will be easily removed when a loadis applied because plasterboard is not particularly strong. Conventionalmethods, such as tapping the wall with a knuckle to determine thelocation of the supporting wooden frame members do not give particularlyaccurate results and require skill. Making a pilot hole through theplasterboard and inserting a piece of bent wire through it into th ecavity likewise is not easy to carry out simply, and although location,e.g. using a small magnet, of the usually iron nails which hold theplasterboard to the wooden structure can be employed, again the resultstend to be rather inaccurate, although this last approach does have theadvantage of avoiding trying to insert a hook where there is already anail underneath. U.S. Pat. No. 5,917,314 discloses a capacitativesensing system for finding wall studs, while U.S. Pat. No. 5,434,500describes a system for marking a position on a partition preciselyopposite a selected position on the other side against which a magneticfield generator is held.

These systems are useful in the building trades, but are not adapted foruse in situations where dimensions are subject to tight tolerancelimits, some of which are particularly critical in manufacture. Forexample, in the manufacture of aircraft, a widely used technique is theapplication of a metal plate or skin to an underlying frame, for examplemade of ribs or spars. In order to ensure a firm connection between theskin and the rib or spar, a technique commonly employed is that offastening the two together, e.g. with a rivet or special fastener. Inorder to do this, apertures in the skin and the rib or spar need tocoincide and this coincidence needs to be particularly accurate since ifthere is inaccuracy, rivetting may be rendered more difficult, or evenimpossible and inadequately-fitting or mis-applied rivets can becomeloosened when the aircraft is in service leading to potentiallycatastrophic failure. Accordingly, the requirements for accuratematching of the hole in the skin with the hole in the rib or spar arevery stringent and the penalty for inadequate accuracy may well be thefailure of the finished assembly to meet the required rigorous safetystandards, leading to the entire assembly having to be recycled.Although if the rib or spar has pre-formed holes, it is notionallypossible to use each of those holes as successive guides for makingholes in an applied skin, this is usually awkward and sometimespractically impossible for reasons of space, and inaccuracies creep in.Additionally, drilling a hole through the skin from inside does notalways provide accurate alignment of the hole in the skin, so that itsaxis runs exactly perpendicular to the surface of the skin. This is aparticular problem where the skin is varying in thickness, e.g. taperingfrom a thick to thin section. Working from the outside, however, i.e.working with the skin between the operator and the spar or rib meansthat the positions of the holes cannot be seen. Attempts to usetemplates to overcome this have not been successful.

The present invention seeks to overcome this problem and to provideapparatus for the detection of a non-visible object, quickly and veryaccurately. It should be noted that the term “object” as used herein isintended to cover a very wide variety of possibilities, including, inparticular, a hole.

Accordingly broadly to the present invention, there is provided a methodof locating an object lying behind an opaque surface rendering theobject non-visible which comprises providing in the neighbourhood of theobject a variable strength magnetic field, sensing the magnetic fieldstrength at a plurality of positions relative to the object using anarray of Hall effect magnetic sensors, the array of Hall effect sensorsbeing associated geometrically with a machining guide, such that themachining guide and the array of sensors are fixed positionally onerelative to the other, interrogating the sensors to determine the valueof the field strength at at least the majority of the sensors, analysingthe sensor responses to determine the displacement between the objectand the machining guide, and moving the array and machining guide to aposition in which the displacement is a minimum.

Using such an approach, the location of the object behind the opaquesurface can be rapidly and easily determined and when the displacementis a minimum, the machining guide is then located adjacent the surfaceat that point of the surface immediately and centrally overlying theobject in question. The position of the array and machining guide canthen be fixed, e.g. by locking the array on the surface, whereafter themachining guide, for example a guide tube, can then be used to guide,e.g. a drill to make a hole in the opaque surface precisely locatedrelative to the non-visible object. Locking of the array on to thesurface can occur e.g. via vacuum pads.

The present invention accordingly also provides apparatus for locatingnon-visible objects positioned behind an opaque surface, which apparatuscomprises means to generate a variable strength magnetic field, a basemember adapted to be placed on or against the surface, means in the basemember defining a machining guide, an array of Hall effect sensorslocated relative to the machining guide, and means for collecting andanalysing outputs from at least some of the sensors to provide anindication of the variation of the magnetic field associated with theobject relative to the position of the base member.

The base member is preferably adapted to be removed across the surfaceto enable the machining guide to be aligned with the object. Theapparatus preferably includes fixing means adapted to lock the relativeposition of the base member and the object relative to one another.Preferably the means for analysing includes a visual display meansadapted to indicate the location of the object relative to the array ofsensors, and accordingly to indicate when the array is positioned withthe machining guide associated therewith located closest to thenon-visible object.

The present invention is particularly valuable in the technical area oflocating holes, particularly, though not exclusively, in the technicalfield mentioned above, i.e. in fitting an opaque metal skin on tounderlying supporting members in aircraft construction. While it istheoretically possible to detect the presence of a hole in an underlyingspar or strut because the physical properties of the hole differ fromthat of the surrounding material defining the hole, appropriate sensorscan be expensive and the usually necessary alignment and calibration ofan array of them can be complex. In this particular application of themethod of the present invention, however, a simple and highly effectiveapproach is to put a magnet in the hole itself, or locate one relativeto the Hall effect sensors and locate a ferromagnetic material, e.g. asoft iron disc, in the hole.

Conventional alloys used for aircraft construction are predominantlyaluminum alloys which are non-ferromagnetic, so the use of a smallcylindrical magnet enables very clear and defined signals to be obtainedfrom an array of Hall effect sensors, even if the skin is thick, e.g. upto 70 mm thick. Other materials may be even thicker—e.g. carbon fibrecomposites 70 mm or more thick.

As noted above, the object to be located behind the opaque skin is ahole in the spar. However, the object may be, for example, a magnetlocated relative to an (unbored) spar using an appropriate jig, so thatwhen e.g. a bore is drilled using the machining guide, it is drilledthrough both skin and spar, but at the desired position on the spar

The array of sensors is customarily a symmetrical array about themachining guide. The number and positioning of the sensors in the arraymay be varied depending upon the degree of precision required as well ason the type of sensor. A particularly preferred approach is to use acruciform array of sensors with a plurality of sensors located spacedalong the arms of a notional cross, the machining guide then beinglocated at the centre point of the intersection between those arms, asthis needs only relatively straightforward data processing of the sensorsignals. However, in appropriate circumstances, the array may be morecomplex, e.g. 16 sensors×16 sensors arranged in a square grid, or one ormore concentric circles. The processing of the data set from the sensorsmay then be more complex, but the accuracy of positional detection maybe greater.

The visual display providing an indication of the location of the objectrelative to the location of the array is preferably compact and easy tounderstand. A particularly preferred form of display is that of acomputer-driven flat display screen on which are represented inappropriate symbolic fashion the location of the object and the locationof the machining guide. By moving the array and machining guide, thegraphic representations on the screen may be made to coincide. Thedisplay screen may, for example; form part of a conventional laptopcomputer, or a hand-held computing device, often referred to as a PDA.In either case, by combining appropriate programming and interfaceelectronics, the signals from the individual sensors in the array may beprocessed using known techniques to produce the indication on screen. Byappropriate programming, sophisticated features may be introducedrendering the apparatus easier to use, for example automatic re-scalingof the display as the machining guide and object approach coincidence asthe array is moved. Wherein the array is first placed on or against theopaque surface, the location of the object may be displayed relative tothe location of the entire array, and as the array is moved to bringmachining guide and object into close alignment, so the display may bereset automatically to concentrate only on the narrow area around themachining guide, even though the signals from the entire array may stillbe used as desired to calculate the relative positions of array andobject.

The visual display may be dispensed with if the movement of themachining guide and array is under appropriate mechanical control ratherthan manual, for example if the machining guide and array are mounted atthe end of a robotic arm or on an analogous movable base.

Once coincidence has been achieved by moving the array relative to theobject, it is desirable to fix the two temporarily in position onerelative to the other in order to allow the machining guide to be used,e.g. for acting as a positioning jig to enable a mechanical process tobe carried out on the opaque surface, for example drilling a hole at theposition so identified. For this purpose, the apparatus may includemeans for temporarily fixing the array in a position on the opaquesurface, for example by attaching it via actuatable vacuum pads thereto.

The use of vacuum pads is particularly recommended in cases where theopaque surface is not horizontal, a state of affairs often encounteredin the assembly of e.g. large aircraft or aircraft components. In such acase, the base carrying the array of sensors is preferably equipped withvacuum pads which can be subjected to reduced pressure at two discretelevels, one level providing a sufficient holding force to attach thebase member of the array to the surface sufficiently loosely that it canstill be moved around relative thereto, and a stronger holding level atwhich the base member holding the array of sensors is essentially firmlyclamped in fixed position against the opaque surface. Vacuum fixation(or other fixing means) may also be conveniently used to locate adisplay unit, particularly where the display unit is PDA, on a portionof the opaque surface close to the portion under which the object islocated. Operating in this way is possible rapidly to locate, e.g. holesin a spar underneath an opaque wing skin to an adequate degree ofprecision.

Alternatively, the application and fixation of the base member may beachieved by mounting it on a robot arm, and so arranging the control ofthe robot that the base member may be moved to the area of interest,sensing applied to locate the hole and the base member then removed toalign it as desired, whereafter it may then be held firmly in place bythe robot while other actions are effected, e.g. drilling a hole throughthe skin.

The accuracy of performance of apparatus as just described is clearlysusceptible to deterioration on account of sensor ageing. This problemcan be alleviated by providing, for use with the sensor array, some formof standard template of known responsiveness and having means to enablethe base member carrying the sensor array to be accurately andrepeatably coordinated to the template. Using appropriate softwareprogramming, the individual sensor responses can be interrogated whenthe array is positioned on the template and the actual responsescompared with those which should theoretically be produced, or whichhave been produced using the same set-up but in the past, with thecurrent values. The programming of the data capture and analysissoftware may be such as to enable automatic corrections to be applied tocompensate for sensor drift or loss of sensitivity.

By way of further explanation of the invention, and by way ofillustrating how it can be put into practical use, reference is made tothe accompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of a section through a sensorarray located adjacent an opaque metallic skin in turn located adjacenta pre-drilled spar;

FIG. 2 is a diagram showing two alternative approaches to providing thevariable strength magnetic field;

FIG. 3 is a diagrammatic perspective view showing apparatus inaccordance with the invention in use, and

FIG. 4 is a block diagram showing the electronics arrangements in theapparatus.

Referring to FIG. 1, this shows in extremely diagrammatic form, how thepresent invention may be applied to the detection of holes in a spar onthe far side of a metal cladding sheet. For the sake of simplicity, onlyone hole is shown in the flat portion of a spar generally indicated at1, the hole being denoted 2. As shown, a plate 3 (for example an alloyskin for a wing) which is to be attached to the spar is placed againstit.

In order to enable detection of the position of the hole 2, a magnetassembly 4 set in a suitable mounting is located relative to the hole 2from below as shown in FIG. 1.

Located on top of plate 3 as shown in FIG. 1 is a sensor array generally indicated at 10. This array consists of a base member having agenerally flat under surface in which is set a cruciform array ofsixteen Hall effect sensors 12. As seen in the drawing, eight of thesensors 12 are seen spaced to either side of a central sensor 12. Theeight are aligned in a row and the central sensor 12 is, counting fromthe end of the row of sensors perpendicular to the plane of the drawing,the fourth one. Reference number 14 denotes the wall of a cylindricalaperture in the middle of the array.

As will be appreciated, the end of the magnet assembly 4 inserted intohole 2 is the centre location of a generally symmetric magnetic fieldhaving its maximum located in terms of the upper surface of plate 3 atthe point on that upper surface which is precisely aligned with the axisof hole 2. At points on the upper surface of plate 3 more remote fromthat point, the magnetic field strength is less. The magnetic fieldstrength at any point on the surface can be measured using a Hall effectsensor.

The Hall effect sensors 12 are connected via a suitable signal-carryingcable 16 to an evaluation electronics, for example in the form of alaptop computer or PDA.

It will readily be appreciated that if the array 10 is located as shownin FIG. 1 with the hole 14 located coaxially with the hole 2, then themagnetic field strength will be greatest and of equal value at thepositions of the Hall sensors radially closest to aperture 14, with thefield strength detected by each of the sensors more remote from aperture14 being less, and being lowest at the outermost ones.

If the array 10 is shifted from its position in FIG. 1, the fieldstrengths will vary at the individual sensors 12 and the signals fromthem can be appropriately analysed to work out how far the axis ofaperture 14 then deviates from the axis of hole 2. By moving assembly 10to minimise that deviation, the aperture 14 may be aligned with hole 2essentially seen from above as shown in FIG. 1. Aperture 14 may then,for example, have a drilling guide inserted into it, or, for example, amarking implement of some type so as to identify that point on the uppersurface of plate 3 which lies on the axis of hole 2.

FIG. 2 shows diagrammatically two different ways of operating thesystem. Each can be used depending on the particular task involved.

The system shown on the left of the drawing corresponds to the operationas illustrated in FIG. 1, with a magnet 4 one side of an opaque sheet 3,for example an aluminum alloy skin for an aircraft wing, and the Halleffect sensor 12 located on the other.

However, the system may also be operated the “other way round”, as shownon the right of FIG. 2. In that alternative, a magnet 5 can be locatedbehind the Hall effect sensor 12, with a ferromagnetic diffuser plate 6located between them. The magnetic field below Hall effect sensor 12 asseen in the drawing is affected by a ferromagnetic “target” piece 8located the other side of skin 3. This may be a piece of ferromagneticmaterial such as soft iron or, for example, a disk or slug of mouldedplastics material loaded with iron powder or filings. This latterapproach is of particular value in locating holes so that a boreconcentric therewith may be drilled from above as seen in FIG. 2. Eachhole in e.g. an aircraft wing spar, may have such a plastics slug fittedinto it, and these then drop out or are drilled out each time a bore ismade through the opaque sheet of material following the location of thehole and fixation of the base member carrying the Hall effect sensorarray, the machining guide, and, in this case, the magnets 5.

Referring now to FIG. 3, this is a diagrammatic illustration ofapparatus in accordance with the invention which is to be used to locateholes 22 in a pre-drilled aircraft wing spar 20 when it is locatedbehind an aluminum skin 21 which is to be fixed to spar 20 by means ofrivets. Each rivet needs to pass through a hole made in skin 21 andthrough one of the pre-drilled holes 22 in spar 20.

The apparatus consists basically of a main pneumatics and power supplyequipment box 30, a movable drilling guide 31 which, as can be seen, isheld against the skin 21, and which contains the electronics describedbelow and a display unit at a housing 32. Box 30 has a suitable powersupply lead 37 for connecting to a source of electrical power.

On the underside of the unit 31 and accordingly not visible in FIG. 2,is an array of Hall effect sensors. These surround a drilling guide tube33 in an appropriate arrangement, for example cruciform, though otherarrangements can be contemplated.

Unit 31 also carries a couple of vacuum line switches 34 and 35 whichcan be actuated by the user of the system to hold unit 31 very firmlyagainst skin 21, i.e. in fixed position relative thereto, and which canbe adjusted to release the vacuum slightly so that unit 31 can be movedaround on skin 21. Umbilical lead 36 provides air and power to unit 31from box 30.

Before using the apparatus, to locate one of the holes 22 not visiblebehind skin 21, a magnet is placed in one of the holes 22 so that asymmetrical magnetic field spreads out through the skin 21 and its fieldstrength can be detected adjacent the surface of skin 21 visible in FIG.2 by means of the Hall effect sensors on the underside of unit 31. Thosesensors are connected to processing electronics located in unit 31.

By appropriate processing of the signals received from the individualHall effect sensors in the array on the underside of unit 31, thelocation of the maximum magnetic field strength point can be found and,more particularly, displayed on a simple screen display 40 set inhousing 32. Display 40 can be a PDA and housing 32 a docking station.Housing 32 may be affixed by means of a suction cup to the visible sideof ski n 21 at any convenient point. Fixture is effected by a suctioncup actuation lever 41 on housing 32 and the display 40 is connected viaa signal cable 44 with the electronics in unit 31. As can be seen ondisplay 40, the display consists of a pair of concentric circles 45, 46and a (fixed) vertical and horizontal crossbar structure 47. Theelectronics are arranged to show on the screen the position of the pointof maximum magnetic field strength. The crossbar structure 47 ispositioned such that it corresponds to the drilling aperture 33, i.e. asunit 31 is moved, so concentric circles 45 and 46 on the display movelikewise. It is accordingly very straightforward, with unit 32stationary but unit 31 being movable, to move unit 31 into a positionwhere the smaller circle 45 is precisely central relative to thecrossbar structure 47. The positioning is easy and intuitive andanalogous to aligning the target with the crosshairs in a telescopicrifle sight.

Once this coincidence has been achieved, unit 31 may then be clampedfirmly in position on skin 21 and aperture 33 used as a drilling guideenabling a bore to be made in skin 21 which is precisely perpendicularto the surface of skin 21 and which is precisely coincident with thebore 22 in spar 20 which carried the magnet during the positioningprocess. The bore may accordingly be made, unit 31 taken out of the way,a rivet inserted and fixed in position, and the process then repeatedfor the purpose of drilling the next hole in skin 21 to align with thenext aperture 22 in the rib.

FIG. 4 shows a basic diagram of the electronics used in the apparatusshown in FIG. 3.

The dashed boxes in FIG. 4 indicate which parts of the system are housedin unit 31, which on housing 32 and which are housed in box 30. An inputvoltage supply fed via lead 37 is fed via a suitable protection unitagainst over-voltage and over-current to a power supply unit 50. Theprotection unit 50 protects the power supply denoted 51 from anytransients and reverse polarity problems. The power supply unit 51 isbasically designed to generate stable analogue digital power suppliesfor use with the Hall effect sensor array in unit 31 and to provide asystem voltage for powering the digital processing electronics itself.

Located on the output side of power supply unit 51 is a power supplysupervisor unit 52 which is used to monitor the sensor voltage supplyand to indicate, for example by flashing up a message on display 40, ifthere is a problem.

Turning now to the Hall effect sensor array, this is denoted 55 in FIG.3 and the outputs of the individual sensors in the array are fed to amultiplexer 56 and signal conditioning board 57 which is provided withthe necessary electronics to clean and stabilise the Hall effect sensorvoltage measured. The signal corresponding to the voltage selected bymultiplexer 56 is fed to a high resolution analogue/digital converter 58to provide a digital signal corresponding to the Hall effect sensorvoltage and this is fed in turn into a digital signal processing unit 59which stores and processes the digital voltage signals corresponding toeach of the Hall effect sensors in turn. By appropriate programming,this can then calculate the position of the centre of the magnetic fieldrelative to the array itself and it can provide that information via aserial communication interface 60 to the display 40 located on housing32. As noted with respect to FIG. 3, this graphical display presents theposition of the sensor array relative to the mag net in a very easilycomprehendible fashion.

The electronics also has an output interface 61 which can be used tocontrol any external apparatus, for example a monitoring computer or arobot control computer.

1. A method of locating an object lying behind an opaque surfacerendering the object non-visible which comprises providing in theneighbourhood of the object a variable strength magnetic field, sensingthe magnetic field strength at a plurality of positions relative to theobject using an array of Hall effect magnetic sensors, the array of Halleffect sensors being associated geometrically with a machining guide,such that the machining guide and the array of sensors are fixedpositionally one relative to the other, interrogating the sensors todetermine the value of the field strength at at least the majority ofthe sensors, analysing the sensor responses to determine thedisplacement between the object and the machining guide, and moving thearray and machining guide to a position in which the displacement is aminimum.
 2. A method according to claim 1 wherein once the machiningguide is located adjacent the surface at that point of the surfaceimmediately and centrally overlying the object in question, the positionof the array and machining guide is fixed.
 3. A method according toclaim 1 wherein the fixation is effected by locking the array on to thesurface via vacuum pads.
 4. A method according to any one of claims 1 to3 wherein the object is a hole relative to which a magnet or body offerromagnetic material is located.
 5. Apparatus for locating non-visibleobjects positioned behind an opaque surface, which apparatus comprisesmeans to generate a variable strength magnetic field, a base memberadapted to be placed on or against the surface, means in the base memberdefining a machining guide, an array of Hall effect sensors locatedrelative to the machining guide, and means for collecting and analysingoutputs from at least some of the sensors to provide an indication ofthe variation of the magnetic field associated with the object relativeto the position of the base member.
 6. Apparatus according to claim 5wherein the base member is adapted to be moved across the surface toenable the machining guide to be aligned with the object.
 7. Apparatusaccording to claim 5 or 6 and including fixing means adapted to lock theposition of the base member and the object relative to one another. 8.Apparatus according to any one of claims 4 to 6 and wherein the meansfor analysing includes a visual display means adapted to indicate thelocation of the object relative to the array of sensors, and accordinglyto indicate when the array is positioned with the machining guideassociated therewith located closest to the non-visible object. 9.Apparatus according to any one of claims 5 to 8 wherein the array ofHall effect sensors is a cruciform array.
 10. Apparatus according to anyone of claims 5 to 9 wherein the signal display is a computer-drivenflat display screen adapted to represent in appropriate symbolic fashionthe location of the object and the location of the machining guide. 11.A method of locating an object lying behind an opaque surface renderingthe object non-visible which comprises providing in the neighborhood ofthe object a variable strength magnetic field, sensing the magneticfield strength at a plurality of positions relative to the object usingan array of Hall effect magnetic sensors, the array of Hall effectsensors being associated geometrically with a machining guide, such thatthe machining guide and the array of sensors are fixed positionally onerelative to the other, interrogating the sensors to determine the valueof the field strength at at least the majority of the sensors, analyzingthe sensor responses to determine the displacement between the objectand the machining guide, and moving the array and machining guide to aposition in which the displacement is a minimum.
 12. The method of claim11, wherein the object is a hole relative to which a magnet or body offerromagnetic material is located.
 13. The method of claim 11, whereinonce the machining guide is located adjacent the surface at that pointof the surface immediately and centrally overlying the object inquestion, the position of the array and machining guide is fixed. 14.The method of claim 13, wherein the object is a hole relative to which amagnet or body of ferromagnetic material is located.
 15. The method ofclaim 11, wherein the fixation is effected by locking the array on tothe surface via vacuum pads.
 16. The method of claim 15, wherein theobject is a hole relative to which a magnet or body of ferromagneticmaterial is located.
 17. An apparatus for locating non-visible objectspositioned behind an opaque surface, which apparatus comprises means togenerate a variable strength magnetic field, a base member adapted to beplaced on or against the surface, means in the base member defining amachining guide, an array of Hall effect sensors located relative to themachining guide, and means for collecting and analyzing outputs from atleast some of the sensors to provide an indication of the variation ofthe magnetic field associated with the object relative to the positionof the base member.
 18. The apparatus of claim 17, further includingfixing means adapted to lock the position of the base member and theobject relative to one another.
 19. The apparatus of claim 17, whereinthe base member is adapted to be moved across the surface to enable themachining guide to be aligned with the object.
 20. The apparatus ofclaim 19, further including fixing means adapted to lock the position ofthe base member and the object relative to one another.
 21. Theapparatus of claim 19, wherein the array of Hall effect sensors is acruciform array.
 22. The apparatus of claim 19, wherein the means foranalyzing includes a visual display means adapted to indicate thelocation of the object relative to the array of sensors, and accordinglyto indicate when the array is positioned with the machining guideassociated therewith located closest to the non-visible object.
 23. Theapparatus of claim 22, wherein the visual display is a computer-drivenflat display screen adapted to represent in appropriate symbolic fashionthe location of the object and the location of the machining guide. 24.The apparatus of claim 17, wherein the means for analyzing includes avisual display means adapted to indicate the location of the objectrelative to the array of sensors, and accordingly to indicate when thearray is positioned with the machining guide associated therewithlocated closest to the non-visible object.
 25. The apparatus of claim24, wherein the visual display is a computer-driven flat display screenadapted to represent in appropriate symbolic fashion the location of theobject and the location of the machining guide.
 26. The apparatus ofclaim 17, wherein the array of Hall effect sensors is a cruciform array.